What's Mine is Yours
by Obviously Procrastinating
Summary: Derek had a few possessions that he would proudly claim as his (and only his). But maybe it wouldn't be so bad to share with Casey...
1. The Jacket

Casey let out a huff as she continued to pick up the laundry strewn about the house. Lizzie had left her soccer jersey by the front door, Marti had left her pants in the kitchen, and George had left a tie on the couch. ' _What was wrong with these people? Couldn't they pick up after themselves?'_ she thought as she moved to grab Derek's leather jacket. _'Oh, can't wash this one.'_ Right after that thought crossed her mind, she let out a little smirk. Maybe she should wash it just to teach him a lesson. It had been careless of him to throw his red shirt in with everyone else's white clothes and this would be a fitting revenge. An eye for an eye and all that. Plus, if she could shrink it _just_ enough it would be her size. She had been looking for one just like it for a while now.

Casey bit down on her lip as she grabbed the jacket and continued into the laundry room. She felt a little sting of guilt when she threw the jacket in the washer but tried to push it out of her mind. If Derek could ruin her life (or at least her clothes) without showing any remorse, then she should be just as merciless. Newton had made it his third law for a reason.

A few hours later, her plan came to fruition. She was working at her desk, doing homework when she heard Derek yell.

"CASE-Y!" he shouted. Casey felt a smug little smile come to her lips as she went to meet him in the kitchen.

"Yes, Derek?" she said sweetly as she floated into the room. She abruptly stopped when she saw the look on her step-brother's face. It was apparent that he was angry as he stood there holding the shrunken outerwear. This was a new side of him and honestly, she kind of wanted to run away, but stubbornness was in her blood so she stayed.

"Well, Case, I was just wondering why my jacket is suddenly two sizes smaller," he chirped at her, using that same fake sweet tone she had used on him. He could have almost passed for normal if he hadn't been clenching his jaw so tightly.

"Oh," she made her mouth pop into a small 'o'. "Well, I was just trying to be a good sister. You see, I saw the jacket sitting on the floor and it looked like it needed a wash, so I threw it in with the rest of the laundry. It must've shrunk in the dryer." Casey held his gaze and gave him a small shrug. "Oops."

Derek just narrowed his eyes at his step-sister. He knew that by calling her out, he would be showing that he *gasp* actually cared about something. So instead he tried to release some of the tension from his body and just act like it didn't bother him.

"That's, uh, alright. I know that you didn't mean to." It nearly killed him, but he managed to keep his cool. He even added a little smile (it looked more like a snarl but whatever).

Casey felt her expression change to one of surprise. Her eyes widened, and she felt her jaw drop a little before catching it. Never one to completely lose control, she quickly regained composure.

"So, you wouldn't mind if I kept it then, would you?" She said.

Derek pretended to take a minute to think about it. Casey in his jacket? Hell yeah! He wouldn't have to warn off guys anymore. They would all think she was his without him even having to say a word. But then again, did she really deserve it? She had ruined it on purpose after-all. He tapped his chin a few times, drawing out the whole production. He decided he was really going to make her work for it a little.

"Hmmm, I don't know. This jacket really meant a lot to me. I don't know if I could just give it to someone who 'accidentally' ruined it."

Casey straightened her spine as her inner negotiator came out. She was going to have this jacket dammit. Even if she had to rip it out of his cold, dead hands.

"It doesn't even fit you anymore," she quickly pointed out.

"Meh. I could still save it for my future son. Or I could give it to Edwin." His eyes twinkled devilishly. He always knew that kid would come in handy one day. "I know for a fact that he wants it."

Casey started to panic a little. This was the perfect leather jacket. She needed it. It would look so good with her red tank top and her tight, dark jeans. There was no way that she would let Edwin of all people take it from her.

"What do you want? I'll do anything." That was really all he needed. He just wanted to have the upper hand.

"Nah, you can have it Princess. Consider this my good deed for the year. It'll look a lot better on you anyway." He tossed her a quick wink, threw her the jacket and left the room. Casey stood there, jacket limp in her hands, feeling more than confused about the whole situation.


	2. The Chair

It had been two weeks since the jacket incident and Casey was still feeling off-kilter. Derek being nice was unsettling at the time, but even more disconcerting was the fact that his behavior had continued. For example, the first time she had worn the jacket after that afternoon he had only looked at her and nodded in approval instead of calling her ugly. Something strange was going on, but Casey wasn't sure she wanted to know the cause. Sometimes it was better to just let things be.

Their new normal continued until that Friday night. George and Nora were taking the younger kids to see their grandparents which left Derek and Casey at home alone.

"Come on, Case. Let's have a party." The girl rolled her eyes and put her hands on her hips. "We're not having a party. Do you remember what happened last time?" She gave him a moment to reflect on their last oversized gathering and the epic punishment that had ensued.

"I still think we should have a party." Apparently, getting in trouble again didn't matter much to him. Casey let out a deep sigh. "How about a movie night instead?" Derek quickly thought about this. Sure, it wouldn't raise his popularity at school, but it might give him a chance to be closer to Casey in a public-ish setting. He wouldn't admit to himself that he had a crush on her yet, but that didn't stop him for wanting to put some kind of claim on her. It didn't matter if they had a romantic relationship or not. She was his and he wanted _someone_ to know (preferably multiple people, but he would take what he could get). The jacket had gotten people talking, but tonight he could really cement the idea in his friends' minds. He quickly agreed to her idea and went to his room to message Sam and Ralph.

While Derek was hidden away in his room, Casey used the house phone to call her best friend next door. "Hey, Emily. Do you want to come over tonight? My parents are gone, so we're trying to plan a movie night." "Of course! I was looking for something to do anyway. My family is driving me crazy," Emily confided to Casey. "Yeah, I know what that's like," she snorted. "Anyway," Emily continued on, "I'll be over soon." "Okay, see you then!"

Within fifteen minutes, everyone was there. While the guys microwaved popcorn and got drinks in the other room, the girls tried to figure out which movie to watch. "Mmm. The Notebook or The Proposal?" Casey asked. The other girl quickly picked The Proposal. "I love Ryan Reynolds. He's so cute!" Emily squealed. She slyly shot her eyes over to Casey. "But not as cute as Derek."

Casey felt her cheeks flush as she took in Emily's words. There may be times when she hated her step-brother, but she couldn't deny that there was something devastatingly attractive about him. Her best friend thought it was something physical, always mentioning his eyes or his smile, but she was secretly certain that it was something anchored in his personality. His confidence had always intrigued her, and his unexpected kindness would always be one of her greater weaknesses. But the thing that almost brought her to her knees was his faith in her. He truly believed that she was strong enough to withstand his attacks (which, admittedly, most of the time weren't that harsh) and that she was worthy of his (apparently important) time. It was that unfailing confidence in her that made her attracted to him.

Casey blinked her eyes as if to physically clear her thoughts. "Gross, Em," she chuckled. Her best friend let out a snort and moved to fling herself onto the couch as Casey put in the movie. With everything ready to go, the teen girl started to walk towards the couch before abruptly stopping. A lightbulb could almost be seen turning on over her head as she turned towards the chair instead. This way she could annoy Derek and get some personal space at the same time. Her previous thoughts had left her feeling flustered and frankly she just wanted a little space from the guy who was inconsiderately hogging her mind.

The beginning credits had just finished as the guys walked into the room. Sam and Ralph quickly claimed the spots on the couch beside Emily and left Derek by himself to contemplate his next move. "Scooch," he told his step-sister.

"No! I was here first."

"Fine. Have it your way," he said as he moved closer. If she didn't want to move over, fine. They would be sitting a lot closer than he had expected but he wasn't going to back down. It was _his_ chair after all.

Casey let out a surprised huff as Derek flopped into the seat next to her. His actions had left them pressed together from shoulder to ankle, the exact opposite of what she had wanted. Was it too much to get some space in this house?

Even though she was uncomfortable and wanted space, Casey didn't move over. This was her seat and Hell would freeze over before she let Derek get what he wanted without a fight. Instead she settled in to watch the movie, deliberately ignoring the pleasantly warm body beside her. She could survive one movie… right?

Derek on the other hand was over-ecstatic. Here they were, obscenely close in front of the people who knew them and their complicated relationship best. He was almost certain that Sam knew all about his inner celebration right now, because his best friend sent him a little smirk and a subtle tilt of his head towards the pair. Derek rolled his eyes, but he was weirdly happy that someone had noticed. It looked like he had achieved his goal for the night.

The best part was that his stepsister was now cuddling closer to him. Earlier she had been rigid against him, but as the movie had continued on she had let herself get more comfortable. She may have been overreacting before. Her stepbrother was a colossal pain in the ass, but he was warm and he smelled good, and it felt really good to be next to someone that she wanted. There was a primal sense of satisfaction in being close to the object of her interest. The whole situation left her feeling pleasantly fuzzy. Sleepy and cozy and just completely content. Slowly her eyes drifted shut, the rest of the room fading away.


	3. The Bed

A/N: It only took me 8 months and 3 false starts, but here's the next chapter! The endings not perfect, but I hope you enjoy.

After that night, things began to get easier between her and Derek. He was still a pain in the ass, but his teasing wasn't the same as it used to be. Before it had seemed callous and cruel, but now there was a flirty hint to it. Casey didn't know whether it was her imagination or if something had actually changed. It didn't matter though. She was comfortable with their new normal.

Nora and George had noticed the change too. They were no longer worried about leaving the two alone. This meant that they could take a vacation with the younger kids as they had wanted to for so many years now. As a result, Casey and Derek got the house to themselves for a few days. And hey, if they threw a little party then it wouldn't be the end of the world. Everything just had to be cleaned up by Monday afternoon.

By Saturday night Casey had no idea how they would make that happen. Kids had been pouring in since Friday night and the whole house was trashed as a result. There was food and drinks splattered all over the floor, at least one lamp had been broken, and they owed Edwin a new bed. Casey was five seconds away from having a panic attack when Derek appeared and plopped a drink into her hand.

"What is this?" she asked as she carefully examined the drink.

Derek managed to not roll his eyes. He and Casey may have managed to call a truce, but she would always be a little bit suspicious.

"Birthday cake vodka. You need to loosen up, Spacey," he said as he did a little body roll, trying to make her laugh.

He was only a little bit surprised when she snorted. "Yeah, no thanks. At least one of us needs to be sober." The alcohol sloshed a little as she looked for somewhere to set it.

Derek surprised her when he said "it's fine, Case. I'm good."

And for once, he actually was. He had gotten a little trashed the night before but had decided that Casey needed a night to relax and he knew that she wouldn't do anything fun if at least one of them wasn't sober. So, he had left the drinking to his peers. There were plenty of other nights he could spend drunk and stupid. He didn't know how much time he would have left to see Casey cut loose though.

Even though Casey was suspicious of both her step-brother and the drink in her hand, she took a sip. And another one. She was expecting the vodka to be a lot more toxic than it actually was. This stuff really wasn't even that bad. Eventually one drink turned to two and two turned into three. The drunker Casey got, the giddier she became. Classmates she couldn't stand in the past were now her best friends. She danced until she got dizzy and laughed until her stomach hurt. For one night she felt carefree.

Of course, that all came to an end when her stomach began to turn. Derek reached out to grab her shoulder as he passed.

"You okay?"

Her lips pursed in thought. She definitely wasn't okay, but she wasn't sure she wanted Derek involved either. He wasn't exactly the nurturing type. But if she was going to be sick she didn't want to be alone.

She tilted towards him with a serious look on her face. "I think I'm going to be sick."

Disgust and a hint of concern crossed his face as he stopped in his tracks. She was going to obsess about that hint later when she wasn't about to puke.

"Gross, Case."

It kind of was but she didn't care. She needed him to kick whoever was in the bathroom out and lock the two of them in there. There was a big storm comin her way.

Derek huffed out a little breath as he grabbed her arm. It was exhausting being the responsible one. But there was no one else he would do this for.

"Come on," he said as he began pulling her to the bathroom. They made it there only to find a line about ten people deep.

"Comin' through people. I've got a puker!" Derek exclaimed as he pushed through the crowd. His fist knocked politely a few times, but he got no response. "Hurry up!" He said as he began knocking forcefully. Eventually the two kids in there stumbled out while still readjusting their clothes.

"Thanks," he said dryly before pushing Casey in and following closely behind. By the time he had closed the door, she had already fallen to her knees in front of the toilet, looking like she regretted every decision she had made that night. Yeah, he'd been in those shoes before.

"Do you, uh, need me to do anything," he asked as he rubbed the back of his neck. He had never stuck around when someone was vomiting before. Usually, he was the first to leave when things started to get rough.

"Please stay with me," Casey said as she stared into the bowl in front of her. It hurt a little bit to be so vulnerable in front of him, but it felt worse to be alone.

His voice dropped to a whisper. "Okay, I'll stay."

And he kept his word as she got rid of what felt like a week's worth of food. He just closed his eyes and rubbed her back. Sometimes it felt like he would walk to the end of the Earth for her. What was a little vomit?

A wave of exhaustion washed over her when she was done. She just wanted everyone to go home and let her sleep for the next seventy-two hours.

"You okay?" Derek asked her as he stood up and gave the toilet one last flush. She nodded sleepily and said, "I don't think I'm going to drink ever again." He let out a little snicker.

Casey turned around, still sitting on the floor. "Carry me," she demanded as she raised her arms and wiggled her fingers at him.

If her stepbrother was surprised, he didn't show it. Instead he reached down and scooped her up, pulling her into his chest. "My room or yours?" he questioned.

"Yours," Casey murmured. Derek's feet faltered a little because he hadn't expected that to be her answer. His question had been a joke, but if she wanted his room she could have it. Anything of his that she wanted was hers. Every little thing.

They managed to make it down the hall without falling or running into anyone. When they made it inside Derek's room, he carefully deposited the brown-haired girl on his bed.

"Thanks, Der."

"You're welcome," he murmured back as he tucked her in. In the morning his bed would smell like her, sweet and a little spicy (he wouldn't wash his sheets for as long as that scent stuck around).

Once he was content with her position, he made his way for the door. "I'm going to get rid of everyone, okay?"

"Okay. Love you."

His heart cracked a little with her soft admission. "Love you too," he said as he shut the door. It looked like she would have a claim on his bed too.


	4. The Car

The first few days after the party were awkward. Both teens constantly heard Casey's confession repeating in their minds. Casey was pretty sure she must've done something terrible in her past life to have done something so overwhelmingly embarrassing in the present one. She was usually all for sharing her feelings, but something about that moment felt too tender. Whenever she thought about it it hurt a little. But at least Derek didn't harass her about it.

No, instead he kept his distance. He stayed as far away from her as possible, spending more time practicing hockey and actually working on his class work for once. It was scary. Casey spent most of her time hiding and wondering if she had ruined the tentative peace that had been building between the two of them.

Eventually they would have to interact with each other though. You couldn't live with someone and avoid all signs of them existing. Their paths crossed again on a Thursday a few weeks later. Both of them needed the car, Casey for a date and Derek to just drive around and clear his thoughts. Of course neither of them could budge. It was just against their natures (and maybe they were just done missing each other (but don't tell anyone)).

"Derek," Casey's muffled voice came through the door. He opened it to find her all dressed up and pretty. "I know it's your day to have the car but I really _really_ need it. Can I please have it tonight?"

His eyebrow began to climb up his forehead and his eyes narrowed. Something was off.

"What do you need it for?" he asked suspiciously.

Casey's eyes lowered to look at the ground. "I have a date tonight," she mumbled.

That was enough to make Derek see red. Casey never dated. She especially never dated during the summer when she finally had time to do all the nerd things she didn't have time to do during the year.

"No, you can't. I was literally just about to leave. It's my night, Casey," he ground out. And maybe it was a little unfair, but it also seemed unfair that she was dating. He didn't think that this was something that had just popped up. It seemed like something that she had sought out in order to ignore the moment that had passed between them.

"Please," she sighed out. It killed her to beg Derek, but she really didn't have much of a choice at this point. Scott couldn't pick her up and she wasn't about to be trapped in this house walking on eggshells any longer. She was an adult and she would deal with the consequences of her actions. Even if dealing with those consequences meant going on a date with someone else in order to put the awkwardness between her and Derek behind her.

"No," he said precisely before sighing in concession. "But I will take you wherever you need to go."

He was going to blame this hair-brained idea on hope and anger. There was something alluring about Casey being forced to spend time with him in order to get what she wanted. And maybe if they spent enough time together, she would stop being so damn timid around him. He could stop running away if she made it clear that she didn't hate him as much as these past few weeks had made it seem. Maybe a little time together would do the trick.

Casey clenched her teeth before looking back up and meeting his eyes defiantly.

"Fine. I'll be waiting in the car."

She spun on her heel and walked away. Derek had to roll his eyes. This girl, always so headstrong when it came to him. If she would let her guard down for three seconds she would see just how whipped he was for her. She didn't need to always be perfect. He liked her the way she was.

When he got to the car she had her arms crossed and was looking out the passenger window. His breath caught in his lungs when he saw her. But he quickly pushed through it and opened the door.

"Where are we going, princess?" he asked with as much sarcasm as he could muster.

"Drop me off at the mall," Casey muttered back. Derek quickly became miffed. He was going out of his way to drive her where she needed to go. Couldn't she at least be polite? Sure, he didn't let her have the car, but it was his night. He shouldn't have had to.

"Say please."

Casey finally gave him her attention.

"Excuse me?" she asked.

"Say please or I'm not driving you anywhere," Derek stubbornly stated.

The fire in her eyes grew brighter.

"Oh, _please_ Derek. _Please_ take me to the mall so that I can go on my date. _Please_ drive me around so that you can act like I owe you a favor later. I know for a fact you had nothing going on tonight. I could have driven myself, but you," she punctuated with a stab at his chest, "decided that you just had to take me. Take your please and shove it."

This was getting out of hand a lot faster than he had thought it would. He had only wanted to push her buttons a little, get her to stop ignoring him. But things had gone past friendly ribbing straight into hatred town. Then again, he shouldn't have expected anything else. They knew how to push each other's buttons.

Still he knew when it was time to back off a bit.

"Dang, Spacey. That's a little harsh. I just wanted you to use your manners," he finished with puppy dog eyes.

Casey rolled her own at him.

"Please just start the car."

He complied with a grin. They hadn't gone back to friendly but at least they were back to middle ground.

He contemplated how to bring the conversation back around to their confessions. You wouldn't think that it would be hard. All he had to say was something along the lines of 'did you mean it'. She had been drunk but he knew Casey. She wouldn't forget saying something like that. He also knew that he'd have to be a bit more delicate with his delivery if he was going to get what he wanted from her.

"So," he started off nervously. "That was some party wasn't it?"

Casey cut her eyes over to him. "Our party?" she asked cautiously. Her voice was guarded and Derek knew that she knew what he was talking about.

He nodded. "Yeah, our party. You sure had a lot of fun," he said as he let his eyes slide over to her. She was already looking at him.

"Keep your eyes on the road," Casey reprimanded him as her cheeks burned. She felt weird that he had caught her watching him. Usually she was able to keep her creeping to a minimum but after avoiding him for the past two weeks she was dying to take in as much of him as possible.

"I guess I had a good time," she slowly said unsure what he was trying to get at. She knew Derek almost as well as she knew herself and she knew that he would do anything to avoid anything potentially uncomfortable. To her the whole "I love you" situation went so far past uncomfortable it bordered on traumatic. The last time she had felt so shaken up about something was when her dad had visited them in their new house. They were both moments she had considered unbearable failures. At least Derek had said I love you back. It was that sole fact that gave her comfort.

"Me too," he replied. It did something strange to her the way he said. It was soft and deep and she could fall in love with that voice. It made her uneasy.

Her hands ran nervously over the cracked seats. She wanted to change the subject so badly, but at the same time she wanted to push it further. It might hurt but at least she would have an answer.

"Derek?" She waited until he gave a sign that he had heard her. Casey took a deep breath. This was her chance. "Did you mean what you said?"

Derek slammed on the brakes before he remembered that they were on a road. Thankfully they were still in their subdivision and there was no one behind them.

"Derek!"

"Give me a little warning next time, Case." He knew he was avoiding the question, but he didn't think he would have to be the one who answered it. He thought he would be asking her how she felt.

"Well," he started hesitantly. "It depends on what I said." Mentally he was slapping himself. Now was not the time to be flippant. He realized that, he swore he did. But that didn't mean he wanted to admit it. Especially not when that would have given Casey all the power.

"The part where you told me you loved me," she responded dryly.

He took in a deep breath of his own and decided to drop the dumb act. After pulling to the side of the road and stopping he began.

"Yes, Casey. I meant it when I said I love you. I've liked you for a while now, but I realized that I love you a few weeks ago. It's hard not to," he gave her a little grin. "You're weird, but you're my kind of weird."

Casey would've rolled her eyes, but they were too busy tearing up. Was it the perfect moment that she had always hoped for? Absolutely not. For starters she would've hoped that her significant other wouldn't call her weird in his confession. It also wouldn't be in a smelly old car on the side of the road. But hey, sometimes life didn't go according to her plans and she was starting to figure out that that was when the best things happened.

She wiped at her eyes. "Cool," she managed to squeak out.

"Oh, I just get a cool? And you're crying?" Derek teased her. "I'm out here pouring out my heart to you and all I get is a cool and some tears. Damn, Case. That's cold." Derek hoped that he sounded calm and not totally panicked. He couldn't take back his confession if she decided that she didn't feel the same.

"No, no! I love you too. I just wasn't expecting this today," she finished.

The two teenagers sat in silence for another minute while she pulled herself together. With a final sniffle she asked, "does this mean I can drive us home?"

Derek wanted to laugh. She had taken over everything else, why wouldn't she take the car too?

"Knock yourself out." They both got out and made the switch.

"Hey, Derek?" Casey asked. "I really do love you."

Derek rolled his eyes and smiled. "Just drive, Spacey."


End file.
